A rotor of rotary machine supported by an electromagnetic bearing using attraction type electromagnets as a bearing is typically controlled as will be described below. Electromagnetic coils are arranged along one axis on the right and left sides of the rotor and when the rotor is displaced to the right, a control current flows into one electromagnetic coil on the left side to exert attractive force so that the rotor is forcibly deflected to the left. Conversely, when the rotor is displaced to the left, a control current flows into the other electromagnetic coil on the right side to exert attractive force. In this way, the rotor is servo-controlled such that responsive to displacement of the rotor to the right or left, the control current is passed to the electromagnetic coil on the side opposite to the displacement and attractive force generated by that electromagnetic coil brings the rotor into the center position.
U.S. Pat. No. 4,128,795 discloses a prior art electromagnetic bearing control apparatus of this type, which is schematically illustrated in FIG. 14 of this application.
In this known apparatus, detected x and y-component signals of displacement (hereinafter simply referred to as displacement signals x and y) are differentiated to produce x and y-component signals of velocity (hereinafter simply referred to as velocity signals x and y) and the velocity signals x and y are added to the displacement signals x and y at blocks 11 and 12, providing ax+bx and ay+by. The displacement signals added with the velocity signals are passed through a rotation synchronous tracking filter 7 (triggered by a pulse signal) to extract only rotation synchronous components of the displacement signals and velocity signals, x.sub.o =ax.sub.N +bx.sub.N and y.sub.o =ay.sub.N +by.sub.N, which are respectively added with the displacement signals x and y processed by being passed through control circuits 6 and 8, respectively. The sum signals are supplied to electromagnetic coils 2 and 3 through associated amplifiers 9 and 10 in order to control only a rotation synchronous unbalanced vibration. Thus, the bearing stiffness can be adjusted by changing the magnitude of the displacement signals and the bearing damping can be adjusted by changing the magnitude of the velocity signals.
Disadvantageously, the prior art control apparatus requires that the velocity signals be produced from the displacement signals by means of the differentiation circuits, thus complicating the circuit design of the apparatus. Further the addition of the rotational synchronous components of displacement and velocity extracted from the displacement signals x and y and velocity signals x and y by means of the rotation synchronous tracking filter to the displacement signals x and y prevents the rotation synchronous unbalanced vibration from being sufficiently suppressed, due to not to make enough differential signal increase.